A solar cell utilizing sunlight is receiving attention as an energy source that can substitute for a fossil fuel, and various types of studies have been exerted. The solar cell is one type of photoelectric conversion apparatus which can convert light energy into electric energy.
The solar cell which utilizes a pn junction of a semiconductor is most widely distributed, but, since it is necessary to conduct a step of manufacturing a semiconductor material of high purity or a step of forming the pn junction, there is a problem in that an economical cost and an energy cost in a manufacturing process are high.
On the other hand, a dye-sensitized photochemical cell which utilizes a light-excited electron transfer has been proposed by Graetzel et al. (Japanese Patent No. 2664194; J. Am. Chem. Soc. (1993), 115, 6382 to 6390; Nature (1991), 353, 737; and the like) and is expected to be the solar cell of the new generation capable of being produced at low cost by using a low-priced material.
FIG. 6 is a schematic cross-sectional diagram showing an example of a conventional representative dye-sensitized photochemical cell. The dye-sensitized photochemical cell is primarily constituted by a transparent substrate 1 such as glass, a transparent electrode (negative electrode) 2 comprising a transparent conductive film of, for example, ITO (Indium Tin Oxide), a semiconductor layer 3, a photosensitizing dye 4 adsorbed on a surface of the semiconductor layer 3, a counter electrode (positive electrode) 6, an electrolyte layer 5 sandwiched between the semiconductor layer 3 and the counter electrode 6, another substrate 7, a sealing material 8 and the like.
As for the semiconductor layer 3, a porous article prepared by sintering fine grains of titanium oxide TiO2 is used in many cases. On a surface, on the side of the electrolyte layer 5, of the semiconductor 3, a photosensitizing dye 4 is adsorbed. As for the photosensitizing dye 4, a material having an absorption spectrum in the vicinity of a visible light region such as a ruthenium complex is used. As for the electrolyte layer 5, an electrolyte solution containing an oxidation-reduction system (redox pair) such as I−/I2 (however, actually, I2 is combined with I− and exists in a form of I3−.) is mentioned.
An apparatus as shown in FIG. 6 operates as a cell in which the counter electrode 6 is a positive electrode and the transparent electrode 2 is a negative electrode, when light is incident in the apparatus. A theory of such operation is as described below.
When the photosensitizing dye 4 absorbs a photon which passes through the semiconductor layer 3, an electron inside the photosensitizing dye 4 is excited to undergo transition from a ground state to an excited state. The electron in the excited state is quickly drawn out into a conduction band of the semiconductor layer 3 via an electric bond between the photosensitizing dye 4 and the semiconductor layer 3, passes through the semiconductor layer 3 and, then, reaches the transparent electrode 2.
On the other hand, the photosensitizing dye 4 which has been oxidized by losing the electron receives an electron from a reducing agent (for example I−) in the electrolyte layer 5 to be reduced. The reducing agent (for example, I2) which has lost the electron reaches the counter electrode 6 by a diffusion effect and, there, receives an electron from the counter electrode 6 and is, accordingly, reduced and is back to an original reducing agent.
In such a manner as described above, light energy is converted into electric energy without leaving any change in each of the photosensitizing dye 4 and the electrolyte layer 5.
Most important points for effectively operating a photoelectric conversion element are: efficiently absorbing light; efficiently generating-separating a charge carrier (for example, electron) from the excited state caused by absorbing the light energy; and quickly drawing the thus-separated charge carrier outside as a current.
In the dye-sensitized photochemical cell, light absorption is borne by the photosensitizing dye 4 and efficient absorption can be attained by selecting an optimum photosensitizing dye 4.
Generation and separation of the charge carrier from the excited state is performed at an interface between the photosensitizing dye 4 and the semiconductor layer 3. Namely, while the electron is drawn from the photosensitizing dye 4 in the excited state into the conduction band of the semiconductor layer 3, the photosensitizing dye 4 which has lost the electron stays on the surface of the semiconductor layer 3, to thereby attain the generation and separation of the charge carrier.
However, since a subsequent movement of the electron in the semiconductor layer 3 is relied on a diffusive migration, some electrons which are each combined with a hole in the semiconductor layer 3 or recombined with the photosensitizing dye which has lost the electron at the interface between the semiconductor layer 3 and the photosensitizing dye 4 and, accordingly, can not reach the transparent electrode 2 are generated. Since these electrons can not be drawn out as an electric current, the electrons cause a reduction of energy conversion efficiency.
In an effort to enhance the energy conversion efficiency of the dye-sensitized photochemical cell, studies and developments are in progress in various fields. As far as the semiconductor layer is concerned, besides titanium oxide TiO2, not only oxide semiconductors and the like such as Nb2O5 and ZnO are used, but also a complex form thereof, namely, an electrode made of a tin oxide grain zinc•oxide grain mixture, or another electrode made of a complex in which a tin oxide grain is subjected to a surface treatment by a heterogeneous metal oxide is used. (Newest Technology of Dye-Sensitized Solar Cell, supervised and edited by Hironori Arakawa, Chapters 16 and 17, CMC (2001)). However, a concrete guide line for forming a complex while taking an energy level of the semiconductor into consideration has not yet been established.
Under these circumstances, the present invention has been attained and an object thereof is to provide a dye-sensitized photoelectric conversion apparatus in which an energy conversion efficiency has been enhanced and a manufacturing method thereof.